Treadmill stress testing was carried out according to the Bruce protocol on 14 non-hyperinsulinaemic and 11 hyperinsulinaemic obese children and on 43 age-matched controls. The obese groups were matched for body weight, body composition, physical activity and plasma lipid values. Body composition was calculated on the basis of four skinfold measurements. Exercise duration and physical working capacity corrected for body weight and lean body mass were decreased in the obese children (P less than 0.01). The hyperinsulinaemic obese children had lower physical working capacities (in absolute values and when corrected for body weight and lean body mass) than the non-hyperinsulinaemic obese children (P less than 0.05). The exercise period was not significantly different in the two obese subgroups. While fasting plasma insulin levels showed a significant negative correlation with exercise duration and relative physical working capacity in the obese children, the anthropometric parameters did not. It is suggested that the decreased physical fitness in obese children is further aggravated in those with hyperinsulinaemia.